Ultrasonic Flowmeters

Working Principle

The working principle of the ultrasonic flow meter is based on the transmission
of ultrasonic waves in a medium. One or more ultrasonic transmitting-receiving pairs are mounted in or on the pipe, diametrically opposite to each other. The first pair is placed slightly more downstream than the second pair, so they make a certain angle with the pipe longitudinally.

The main idea behind the principle is the detection of frequency or phase shift caused by flowing medium. The effective velocity of sound in a moving medium is equal to the velocity of sound relative to the medium plus the velocity of the medium with respect to the source of the sound. Thus, a sound wave propagating upstream will have a smaller effective velocity, and the sound propagating downstream will have a higher effective velocity. Because the difference between the two velocities is exactly twice the velocity of the medium, measuring the upstream–downstream velocity difference allows us to determine the velocity of the flow.

Types of Ultrasonic Flowmeters

There are various types of ultrasonic flowmeters in use for discharge measurement:

(1)   Transit time

This type of ultrasonic flowmeter makes use of the difference in the time for a sonic pulse to travel a fixed distance, first against the flow and then in the direction of flow. Transmit time flowmeters are sensitive to suspended solids or air bubbles in the fluid.

Figure above shows two ultrasonic generators positioned on opposite sides of a pipe of flow. Piezoelectric crystals are usually employed for that purpose. Each crystal can be used for either the generation of the ultrasonic waves or for receiving the ultrasonic waves. Two crystals are separated by distance D and positioned at angle ‘ф’ with respect to flow. The transit time of sound between two transducers A and B can be found through the average fluid velocity v:

where c is the velocity of sound in the fluid. The velocity v is the flow velocity averaged along the path of the ultrasound. By taking the difference between the downstream and upstream velocities, we find

(2)   Doppler frequency shift

This type is more popular and less expensive, but is not considered as accurate as the transit time flowmeter. It makes use of the Doppler frequency shift caused by sound reflected or scattered from suspensions in the flow path and is therefore more complementary than competitive to transit time flowmeters.

This technique is more popular in so-called “clamp-on” meters. The Doppler effect occurs with sound as well as electromagnetic waves. When a source or receiver moves in a wave medium, the frequency at the receiver will differ from the frequency at the transmitter. The frequency increases with a movement towards the source and it decreases with a movement away from the source. This is caused by the constant velocity of the wave in the medium. If all velocities in the same direction are counted positively, we can describe the Doppler effect as follows:

Where

f_o: observed frequency for movement

f_s: frequency of the source in rest

c: transmission velocity in the medium

v_o: velocity of the observer with respect to the medium

v_s: velocity of the source with respect to the medium

During the measurement the source (transmitting crystal) and the observer (receiving crystal) are fixed and the fluid is moving. The transmitted signal is only detected if dispersed by moving fluid particles. These particles can be solids or small gas pockets. The Doppler technique only works in liquids that contain enough solids or gas pockets.

The Doppler frequency shift is:

For the Doppler flow measurements, continuous ultrasonic waves can be used. Figure below shows a flowmeter with a transmitter–receiver assembly positioned inside the flowing stream. Here angles are zero. That differential is defined as

Installation requirement:

In general acoustic flow meters need no special requirements regarding installation on or in the process pipe:

-        A vibration-free location is recommended especially when applying the Doppler type flowmeter, as vibrations cause false signals which may fool the electronics.

-          Similar to most flow meters, the measuring pipe must be completely filled with the fluid.

-          A well-developed flow profile is absolutely required for a reliable and accurate measurement. That is why equalization pipes 10 D in front of and 5 D behind the meter are recommended to obtain the given level of accuracy (2%).

-          Pilot valves closely behind the flow meter negatively influence the measurement, especially when cavitation or supersonic velocities occur.

Characteristics:

–        No pressure loss in the pipe.

–        It is possible to measure without making contact with the fluid (“clamp-on”).

–        Only useful for liquids that are acoustically transparent.

–        A small, but not excessive amount of contamination of the liquid is necessary for the Doppler effect. The time-of-flight principle needs as little pollution as possible.

–        Difficult for small diameters, especially when using the time-of-flight principle.

–        For the time-of-flight difference meter the turndown can amount to 1:1,000 and an accuracy level from 1% to 2.5% is possible. For the Doppler type meter the turndown can amount to 1:3,000 with an accuracy level from 2% to 5%.

–        Individual calibration is needed for every medium.

–        Using the Doppler effect, the reading depends on the flow profile.

–        Accuracy levels up to 1% are possible (not when using clamp-on realizations).

–        At present it is also possible to ultrasonically measure the flow of gases, steam and even high-temperature steam.

Questions
GATE 2014
A transit time ultrasonic flowmeter uses a pair of ultrasonic transducers placed at 45° angle, as shown in the figure.

The inner diameter of the pipe is 0.5 m. The differential transit time is directly measured using a clock of frequency 5 MHz. The velocity of the fluid is small compared to the velocity of sound in the static fluid, which is 1500 m/s and the size of the crystals is negligible compared to the diameter of the pipe. The minimum change in fluid velocity (m/s) that can be measured using this system is_________.

GATE 2018
T he average velocity v of flow of clear water in a 100 cm (inner) diameter tube is measured
using the ultrasonic flow meter as shown in the figure. The angle ø is 45 degree. The measured
transit times are t1 = 0.9950 ms and t2 = 1.0000 ms. The velocity v (in m/s) in the pipe is (up

to one decimal place) ___.